The heat pump system of the present invention is designed to operate in one of four different modes with respect to indoor air, namely (1) solar energy source heat pump heating; (2) direct heating; (3) air source heat pump heating; and (4) heat pump cooling. The heat pump system employs a non-reversing refrigeration cycle and utilizes heat exchangers, preferably in the form of a water-cooled condenser and a counterflow chiller in a fluid circuit where an expansion valve responds to the temperature of the refrigerant at the evaporator outlet to form a control capable of operating as close to zero degree superheat as feasible. A combination suction-liquid heat exchanger and suction accumulator protects the compressor from possible flood-back at this low superheat. The non-reversing refrigeration cycle, in the preferred form of the invention, includes a hold-back valve also known as a crankcase pressure regulator to maintain constant compressor suction pressure regardless of evaporator pressure fluctuations due to changing fluid medium temperatures in the chiller.
In the well-known form of a heat pump, compressed refrigerant is evaporated in an outdoor evaporator coil or heat exchanger and thereafter the expanded refrigerant is compressed and passed through a condenser to extract heat from the condensing refrigerant for heating the interior of a building. The refrigeration cycle is reversed during warm weather such that the system operates as an air conditioner. In cold climates, the heat pump must operate at outdoor temperatures below +32.degree. F., sometimes as low as -20.degree. F. At these temperatures, the evaporating temperature of the refrigerant in the outdoor evaporating coil drops to a point where the coefficient of performance of the heat pump is uneconomically low. The coefficient of performance falls to 2.0 or lower. This fails to provide sufficient heating capability.
A non-reversing compression refrigeration cycle is used by the present invention. This eliminates many service problems encountered with reversing heat pumps. In cold climates, solar energy supplies a useful heat source that is limited to daylight hours and favorable atmospheric conditions. At outdoor temperatures of +40.degree. F. or higher, ambient air provides a useful heat source that is not subject to the same limitations as solar energy. These two heat sources can be used effectively in a heating system. The present invention integrates the use of these two heat sources into a heat pump system for various and alternative modes of operation.